3.3 Evolution of plastic strain of thermal treated granite
specimens with axial strain
Plastic strain is the irrecoverable strain in the cyclic
loading-unloading process, which can reflect the damage in specimen to
some extent. Figure 4 illustrates the variation of axial plastic strain
with the maximum axial stain (ε 1u) in each cyclic
loading, it is clear that axial plastic strain non-linearly increases
with the maximum axial strain on a concave type. Axial plastic strain
increases gradually at the initial loading stage, and inflection point
appeared. Finally, axial plastic strain near linearly increases with the
maximum axial strain in each cycle at macro-crack forming and shear
slippage stage. With increasing confining pressure, axial plastic strain
decreases, which indicates that confining pressure can retrain the axial
plastic strain to some extent. When T = 600°C, axial plastic
strain near linear increases with the maximum axial strain in each
cycle, which indicates that damage occurred once applied loading. In
conclusion, the damage evolution is influenced by the initial damage
state and confining pressure.
Figure 5 shows the variation of radial plastic strain with the maximum
axial strain, it can be seen that the radial strain is smaller in the
initial stage, and even compressive characteristic appeared, this
phenomenon was also obtained by Xiong et al. (2019). The partial
enlarged drawing of the radial plastic strain is shown in Fig. 5d, the
compressive characteristic is more obvious with increasing temperature.
It indicates that the grain adjusted under triaxial cyclic
loading-unloading compression when there are micro-cracks in specimen.
The compressive characteristic is more unobvious with increasing of
confining pressure, it may be because micro-crack pre-closed when
applied confining pressure. And then radial plastic strain shows
expansion characteristic with increasing the maximum axial strain in
each cycle. It can be also seen that the reversed point from compression
to expansion is backward with increasing confining, it indicates that
confining pressure restrain radial plastic strain.